1. Field of the Invention
The present invention relates to pressure measurement in fluid pumping systems and, more particularly, to the correction of pressure offset errors in such systems.
2. Description of the Prior Art
U.S. Pat. Nos. 4,131,393 and 4,190,375 describe fluid pump mechanisms for high-performance liquid chromatography (HPLC). In HPLC systems, a fluid sample mixture carried in a solvent matrix is pumped at high pressure through a densely packed separation column. The column separates the sample into individual sample fractions which exit the column one after another. The separated fractions flow in succession through a flow cell detector, such as a spectrophotometer, which performs quantitative and/or qualitative measurements of each fraction.
In a prior HPLC pump mechanism, system pressure is monitored with a pressure transducer generating a signal indicative of the monitored pressure for controlling various aspects of system operation. For example, the pressure signal indicates whether the system is operating within a required pressure range for analysis since the system is designed to operate between maximum and minimum pressure limits. Pressures in excess of the maximum can cause permanent damage to the system structure. Pressures below minimum pressures are associated with undesirably low flow rates through the column. In addition, departures design system pressure may indicate pump and system disfunction such as a clogged column or flow line, a flow leak, or a depleted solvent reservoir. Thus, pressure signal levels outside of the predetermined operating pressure range indicate unsafe or incorrect system operation, requiring corrective action.
In the identified systems, the signal representing measured system pressure signal is also employed to control pump operation through an electronic feedback loop. The described electronic circuit automatically compensates for changes in fluid compressibility, wear of seals and valves, and solvent gas emission. Compensation is detained by change in pump speed corresponding to in measured pressure.
Prior art systems have performed well and have found wide commercial acceptance. However, it has been found that changes in temperature and ofther system operating parameters can cause inaccurate measurement of system pressure. For example, monitoring of pump pressure using a pressure transducer, it has been found that the transducer signal varies with temperature and can drift over time. Such temperature variance and drift creates baseline error relative to the reference level of the pressure signal.
Previously, manual adjustments have been used to correct baseline error as described above. Typically, the system pressure is reduced to zero and any offset or deviation found in the zero pressure measurement is compensated by manual adjustment of a measurement device. This is an unsatisfactory arrangement since its implementation requires ceasing operation of the HPLC system and depressurization of the system. This eliminates the possibility of concurrent manual adjustment in system pressure adjustment. Frequent adjustments can require continued system shut-downs. Furthermore operator attention is diverted from other tasks.